Well logging system for determining directional resistivity using multiple transmitter-receiver groups focused with magnetic reluctance material

Abstract

An electromagnetic wave propagation resistivity borehole logging system comprising multiple groups of electromagnetic transmitter-receiver arrays operating at three frequencies. The borehole logging tool component of the system employs eight transmitters and four receivers. The transmitters and receivers are disposed axially and symmetrically along the major axis of the tool to form four group pairs. Each group pair consists of a transmitter-receiver groups axially and symmetrically on opposing sides of a reference point on the tool. Each, transmitter-receiver group consists of one transmitter assembly and two receiver assemblies. Each transmitter-receiver group is operated at two of three operating frequencies which are 100 kHz, 400 kHz and 2 MHz. Some of the transmitter and receiver assemblies are fabricated to yield azimuthally focused resistivity measurements, and to yield vertical and horizontal resistivity in anisotropic dipping beds. The system can be embodied as a logging-while-drilling system or as a wireline logging system.

What is claimed is:  
     
       1. An antenna assembly located on a tubular housing comprising: 
       (a) alternating land members and axial slots wherein  
       (i) an outer radius defines a periphery of said land members,  
       (ii) each of said axial slots is defined by adjacent land members and an inner surface defined by a radius less than said outer radius, and  
       (iii) an antenna pathway traverses each said land member;  
       (b) an antenna which is  
       (i) essentially circular and defines a first plane essentially perpendicular to the major axis of a tubular housing, and  
       (ii) passes through each said land member within said antenna pathway; and  
       (c) at least one magnetic reluctance material insert affixed to said inner surface of at least one of said axial slots, wherein directional response of said antenna assembly is adjusted by varying geometry of said magnetic reluctance material insert within said at least one axial slot.  
     
     
       2. The antenna assembly of  claim 1  comprising a plurality of magnetic reluctance material inserts wherein each of said plurality of inserts is affixed to said inner surface of one said axial slot. 
     
     
       3. The antenna assembly of  claim 2  wherein: 
       (a) said inserts are affixed to said inner surfaces of said slots so that midpoints of said inserts fall on a second plane; and  
       (b) said second plane intersects said first plane.  
     
     
       4. The antenna assembly of  claim 2  wherein volumes of said inserts vary monotonically in adjacent said axial slots. 
     
     
       5. The antenna assembly of  claim 4  wherein midpoints of said inserts fall in a plane that is parallel to said first plane. 
     
     
       6. The antenna assembly of  claim 1  wherein surfaces defining said antenna pathway are coated with highly conducting material. 
     
     
       7. The antenna assembly of  claim 1  wherein surfaces defining said axial slot are coated with highly conductive material. 
     
     
       8. The antenna assembly of  claim 1  wherein said assembly operates as a transmitter. 
     
     
       9. The antenna assembly of  claim 1  wherein said assembly operates as a receiver. 
     
     
       10. The antenna assembly of  claim 1  wherein said magnetic reluctance material insert comprises ferrite. 
     
     
       11. A method for directionally transmitting electromagnetic radiation, the method comprising the steps of: 
       (a) forming an antenna assembly by alternating land members and axial slots, wherein  
       (i) an outer radius defines a periphery of said land member,  
       (ii) each of said axial slots is defined by adjacent land members and an inner surface defined by a radius less than said outer radius, and  
       (iii) an antenna pathway traverses each said land member;  
       (b) energizing an antenna which is  
       (i) essentially circular and defines a first plane, and  
       (ii) passes through each said land member within said antenna pathway; and  
       (c) affixing at least one magnetic reluctance material insert to said inner surface of at least one of said axial slots, wherein directional response of said antenna assembly is adjusted by varying geometry of said at least one magnetic reluctance material insert within said at least one axial slot.  
     
     
       12. A method for directionally detecting electromagnetic radiation, the method comprising the steps of: 
       (a) forming an antenna assembly by alternating land members and axial slots, wherein  
       (i) an outer radius defines the periphery of said land members,  
       (ii) each of said axial slots is defined by adjacent land members and an inner surface defined by a radius less than said outer radius, and  
       (iii) an antenna pathway traverses each said land member;  
       (b) measuring current induced within an antenna by said electromagnetic radiation, wherein said antenna is  
       (i) essentially circular and defines a first plane, and  
       (ii) passes through each said land member within said antenna pathway; and  
       (c) affixing at least one magnetic reluctance material insert to said inner surface of at least one of said axial slots, wherein directional response of said antenna assembly is adjusted by varying geometry of said magnetic reluctance material insert within said at least one axial slot.  
     
     
       13. A electromagnet wave propagation resistivity logging system comprising: 
       (a) a tubular housing;  
       (b) a plurality of transmitter-receiver groups axially spaced along said tubular housing and wherein  
       (i) each said transmitter-receiver group comprises at least one transmitter assembly and two receiver assemblies,  
       (ii) each said transmitter-receiver group is operated at two of three operating frequencies, and,  
       (iii) the lowest of said operating frequencies is about 100 kHz, and wherein  
       (iv) each transmitter assembly and each receiver assembly comprises an antenna assembly which comprises,  
       alternating land members and axial slots wherein an outer radius defining the periphery of said land members is essentially the same as the outer radius of said tubular housing, and each of said axial slots is defined by adjacent land members and an inner surface defined by a radius less than said outer radius,  
       an antenna pathway which traverses each said land member,  
       an antenna which is essentially circular and defines a first plane essentially perpendicular to the major axis of said tubular housing, and which passes through each said land member within said antenna pathway; and wherein  
       magnetic reluctance material inserts are affixed to said inner surface of said axial slots, and directional response of said antenna assembly is adjusted by varying geometry of said magnetic reluctance material inserts within said axial slots;  
       (c) electronic circuitry which  
       (i) activate said transmitter assemblies in each of said transmitter-receiver groups,and  
       (ii) record induced signals in each said receiver assembly of each said transmitter-receiver groups; and  
       (d) a processor programmed to combining said recorded induced signals to yield a measure of resistivity of material in the vicinity of said tubular housing.  
     
     
       14. The system of  claim 13  wherein said transmitter-receiver groups are symmetrically disposed on opposing sides of a reference point on said tubular housing. 
     
     
       15. The system of  claim 14  wherein two of said operating frequencies are about 400 kHz and 2 MHz. 
     
     
       16. The system of  claim 13  further comprising: 
       (a) a conveyance unit disposed at the surface of the earth; and  
       (b) a conveyance member with a first end connected to said tubular housing and a second end connected to said conveyance unit thereby allowing said conveyance unit to convey said tubular housing along a well borehole.  
     
     
       17. The system of  claim 16  wherein: 
       (a) said conveyance unit is a drilling rig; and  
       (b) said conveyance member is a drill string.  
     
     
       18. The system of  claim 16  wherein: 
       (a) said conveyance system is a wireline draw works; and  
       (b) said conveyance member is a wireline.  
     
     
       19. The system of  claim 13  wherein surfaces defining said antenna pathway are coated with highly conducting material. 
     
     
       20. The system of  claim 13  wherein surfaces defining said axial slot are coated with highly conductive material.